There is a need for very narrow bandwidth bandpass filters for use in wireless communication devices, especially for those operating in Industrial, Scientific and Medical (ISM) bands. The ISM 2.4 band in particular is becoming increasingly crowded with microwave ovens and competing wireless devices. The extraneous signals from microwave ovens and other wireless devices interfere with the proper reception of wanted signals by saturating the input of the receiver. These interferences can be managed by increasing the linearity of the receiver, which requires more current. A more linear receiver will be able to amplify both the small, wanted signals without distortion, as well as the large, unwanted interferers. This more linear receiver will be more sensitive to the wanted signals. However, this solution is not desirable for battery operated wireless communication devices since the extra current shortens battery life. A better solution for battery operated wireless communication devices is to use band pass filters to exclude the bulk of the unwanted signals, which allows for a more power efficient operation, and thus, results in longer battery life.
Unfortunately, conventional active filters do not have the frequency response capable of narrowband operation at these frequencies. It is typical to use active filters after the signal is down converted so that the filter operation is at a much lower frequency. This leaves the electronics between the antenna and up to and including the down converter (also known as the “frontend”) susceptible to the unwanted interference. The solution here lies in increasing the bias current into the frontend, but at a cost of reducing battery life.
Currently available passive filters in the ISM 2.4 band also do not have the capability of reducing the bandwidth of the front end. Since battery operated wireless communication devices are preferably small, inexpensive products, the filter types available are limited to ceramic, Surface Acoustic Wave (SAW) and Film Bulk Acoustic Resonator (FBAR). A conventional filter based on ceramic with less than 20 MHz bandwidth is not practical, and a conventional filter based on SAW or FBAR is not possible. The problem for a bandpass filter based on SAW or FBAR is that when the resonators are combined into a filter network, each resonator loads the other resonators. If the resonant frequencies of the resonators are too close, the filter insertion loss soars, making the filter unusable. This phenomenon is illustrated in FIG. 1, which shows that as the bandwidth is reduced, the insertion loss is increased.
In view of these concerns, there is a need for a narrow bandwidth bandpass filter network and method for bandpass filtering signals in a narrow bandwidth of a desired frequency band, such as the ISM 2.4 band.